Electronic circuit for optimizing switching losses in current-driven power devices

ABSTRACT

An electronic circuit is for optimizing or reducing switching losses in current-driven power devices and includes a switching power device connected to an electric load. The power devices has at least one control terminal arranged to receive a predetermined drive current value produced by a first current generator. The control terminal also receives an additional drive current portion produced by a second independent current generator. Advantageously, the electronic circuit includes a control circuit for controlling a switch connected between the second current generator and the control terminal of the switching power device during the turn-on and turn-off phases of the power device.

FIELD OF THE INVENTION

The present invention relates to an electronic circuit for reducingswitching losses in current-driven power devices. The invention relates,particularly, but not exclusively, to an electronic circuit comprising apower device connected to an electric load and having at least onecontrol terminal arranged to receive a predetermined drive currentvalue.

BACKGROUND OF THE INVENTION

In most electronic applications, and especially those includinghigh-frequency operations, there is a need for reducing or minimizingthe power losses during the switching phases of the power devices. Thisis desired to keep the operating temperature of the power devices as lowas possible.

The problems involved in reducing switching losses at turn-off areindependent from the kind of load that is connected to the power device.On the contrary, in applications involving just resistive loads, andwith currents that will quickly attain their maximum value, the turn-onphase must be considered.

High reactivity is demanded in the power device at turn-on, to preventthe appearance of elevated currents and/or voltages. It is in thisperspective that the art has long been looking for circuit schemes thatcan enhance or optimize both the dynamic saturation and the performanceof power devices at turn-on and turn-off.

Many of the power devices formed by discrete components and intended fordriving electric loads comprise bipolar transistors. A speed-upcapacitor connected in parallel with a base resistor Rb is commonly usedfor driving the bipolar transistors. At the turn-on phase, the capacitorsupplies a current pulse that, once added to the current from the baseresistor, will optimize the saturating volt age of the powertransistors. At the turn-off phase, the speed-up capacitor will output acurrent pulse from the base terminal of the bipolar transistor. Thiscurrent pulse contributes to dissipating the stored charge, thusshortening the storage time.

Integrated driver circuits for driving electric loads include finalpower stages that are controlled by currents that are proportional tothe output currents of the power stages. A maximum current is flowedthrough these circuits only when needed, such as when a power elementarranged to produce a current for an inductive load is driven by a drivecurrent that is proportional to the load current.

The approaches just described have certain limitations, either wherediscrete components or integrated circuits are provided. For instance,when the signal originates from a resistive load, whereby the currentwill rise to its final value immediately, the driving method based onthe proportional current cannot be applied. Also, when large currentsare flowed through the circuit, the approach based on the use ofdiscrete components will require a large speed-up capacitor and,therefore, result in a substantially higher device cost.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an electronic circuiteffective to reduce or optimize switching losses in current-driven powerdevices, such a circuit having relatively simple structural andfunctional features to allow enhancing the turn-on and off phases of thepower device to be driven, thereby addressing the limitations of theprior art.

The circuit in accordance with the invention provides a current stepadditional to the drive signal and effective to lower the voltage acrossthe power device during the switching phases of the power device. Oneembodiment of the invention relates to an electronic circuit forreducing or optimizing switching losses in current-driven power devices.This circuit may comprise a switching power device connected to anelectric load and having at least one control terminal arranged toreceive a predetermined drive current value from a current generator,and a structurally independent current generator producing a furtherdrive current portion to supply to the control terminal. The circuitalso includes a controlled switch between the independent currentgenerator and the control terminal, and a control circuit controllingthe switch during the turn-on and turn-off phases of the power device.

Another embodiment of the invention also relates to an electroniccircuit for reducing switching losses in current-driven power devices.This circuit may include a power switching device connected to anelectric load and having at least one control terminal receiving a firstdrive current value produced by a first current generator, and a firstcontrolled switch between the first current generator and the controlterminal. The circuit may also include a second current generatorproducing a second drive current value to be supplied to the controlterminal, a second controlled switch between the second currentgenerator and the control terminal, and a control circuit controllingthe first and second switches during the turn-on and turn-off phases ofthe power device.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and advantages of a circuit according to the invention aremade apparent by the following description of an embodiment thereof,given by way of non-limiting example with reference to the accompanyingdrawings in which:

FIG. 1 shows the circuit of this invention in schematic block form;

FIG. 2 shows a further embodiment of a circuit according to the presentinventions

FIG. 3 shows schematically a detail of the circuit of FIG. 2;

FIG. 4 is a detailed circuit diagram illustrating an embodiment of theinventive circuit;

FIGS. 5A, 5B and 5C are respective voltage and current plots versus timefor signals present in the circuit of FIG. 4;

FIGS. 6 and 7 are comparative plots of voltage and current signalspresent in driver circuits, with and without the circuit portion of thisinvention, respectively; and

FIGS. 8 and 9 are further comparative plots of voltage and currentsignals present in the circuit of this invention during the turn-offphase.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference to the drawings, an electronic circuit according to thisinvention and adapted to reduce or optimize switching losses incurrent-driven power devices 2, is shown generally at 1 in schematicform. In a broad sense, the power devices 2 are considered hereinelectronic components capable of switching on/off electric loads 4, thussupplying to the load relatively large currents during the turn-on phaseand cutting off such currents during the turn-off phase.

An electric load is considered herein an electric or electroniccomponent of whatever kind but being activated by a current flow or by apredetermined voltage applied to its terminals. In this invention, theelectric load may be a resistive or an inductive load, e.g. a halogenlamp 4.

The power device 2 is represented schematically in FIG. 1 by a blockPower Stage, which is connected between a supply voltage reference Valimand a ground voltage reference GND. A control terminal 5 of the powerdevice 2 receives a current control signal.

Advantageously in this invention, the terminal 5 will receive a first orcontrol current Ib and a second or additional integrative current Ib′.The second or additional current Ib′ is illustratively supplied from astructurally independent generator 6 through a controlled switch S.

The first current Ib is supplied to the terminal 5 from anothergenerator 8 through a controlled switch N. A control circuit 9 isarranged to drive both switches S and N. This circuit 9 will control theduration and amplitude of the current pulse so as not to deteriorate theperformance of the power device 2 at turn-off.

To enhance or optimize the performance of the circuit according to theinvention, the control circuit 9 will, at turn-off, open the switch Supon the device 2 being turned off. This thereby supplies a currentvalue Ib that is suitably smaller than the value supplied at turn-on. Inthis way, the charge storing and voltage drop effects can be attenuatedwithout adversely effecting the power device saturation.

A further embodiment will now be described with reference to FIG. 2,wherein the invention is applied to a specific electronic transformerfor driving a load 3 represented by a halogen lamp 4. The power device 2is incorporated in a half-bridge structure including a pair of driveelements, specifically a high-side driver component 10 and a low-sidedriver component 11. These are connected in series with each otherbetween the supply Valim and a ground reference GND.

An interconnect node X between the components 10 and 11 is connected tothe halogen lamp 4. A first winding 12 is provided between the node Xand the high-side component 10, and a second winding 13 is providedbetween the node X and the low-side component 11. A load current Iflowing through the lamp 4 is switched alternately, preferably at therate of 30 to 50 kHz, by the half-bridge legs.

A high voltage supply for the lamp is derived from an external AC sourcethrough a diode bridge rectifier 15, itself connected to a diac elementcomprising a parallel-connected capacitor C1 and a Zener diode Z1. Afirst circuit RC1 is inserted between the high voltage supply and groundto provide the voltage value Valim to be applied to the low-sidecomponent 11. A second circuit RC2 is connected between the high voltagesupply and ground to provide a second voltage value to start-up a diaccomponent D. Once the diac D oscillation has been started, a seriesarrangement of a diode D1 and a resistor R3 is used to turn-off the diacD. A third circuit RC3 is connected between the high voltage supply andthe load 3, to supply the voltage value Valim to the high-side component10.

A resistor R4 is connected between ground and the load 3, and acapacitor Coscill is connected between the node X and the high-sidecomponent 10. A further capacitor Coscill is connected between theground and the low-side component 11. A pair of steadying capacitors, C6and C7, connect the supply voltage to ground through one end of the load3.

Shown schematically in FIG. 3 is the internal structure of each of thepower components, 10 or 11, and the control block 9 associated with eachof them. Each component 10, 11 comprises basically an emitter-switchingdevice.

A low voltage MOS power transistor M has its conduction terminalscoupled to two terminals C and S, across which a potential drop Vcoccurs. The control terminal of this transistor M is driven by agate-driver block 17 that gets a voltage from the supply voltage Valim.The block 17 receives a voltage enable signal Vsec from a terminal IN ofthe power component.

A pair of bipolar transistors, Q1 and Q2, are interconnected in aDarlington configuration and placed between the terminal C and the MOStransistor M. The base terminal B of the first, Q1, of the bipolartransistors is applied a current signal through a series of a resistorpair, Rb1 and Rb2. The series resistor Rb2 in the pair can beshort-circuited through a controlled switch K.

An over current block 18 (Dynamic Vc) has an output connected to thebase terminal B. This block 18 is supplied by the voltage value Valimand controlled from a frequency control block 19. This block 19 alsodrives the switch K and the control terminal of the MOS transistor M.This block 19 is supplied by the voltage value Valim and connected toone end of a capacitor Coscill. The gate driver block 17 functions todeliver a useful signal to the gate of transistor in the presence ofsecondary.

By charging a capacitor, the frequency control block 19 sets theconduction time length or duration of the power transistor M, and manages the over current generating block 18. The turn-on duration of thepower device 2 is controlled by a comparator 20, shown in FIG. 4, whichcomprises MOS transistors M21, M22, M23, M24. This comparator 20 ispowered through a transistor M20, which represents a second leg of acurrent mirror 21 that also comprises a MOS transistor M16, the latterbeing enabled in the presence of secondary current. The comparator 20serves to compare the charge in the capacitor Coscill—as derived fromthe current of a transistor M33, which transistor mirrors transistorM16—with the potential across a resistor R2.

When a voltage Vcoscill exceeds a voltage drop VR2, the output from thecomparator 20 enables an N-channel transistor M34 arranged to turn off adouble inverter 22, 23 comprising transistors M18, M19, M25, M26. Thisdouble inverter forms the gate driver block 17. A second comparator 24,comprising transistors M12, M13, M14, M15 and being powered through afurther transistor M11, will compare the signal at the capacitor Coscillwith the potential at the resistor R5. The output from the secondcomparator 24 is initially low, and enables, through the double inverter22, 23, a PMOS transistor arranged to provide the over current Ib′.

When the voltage Vcoscill exceeds a voltage drop VR5, the PMOStransistor is disabled. By suitably selecting the value of VR5 relativeto VR2, the duration of the current supply with the additional currentIb′ can be set with respect to the turn-on duration of the power device2.

A third comparator 25, comprising transistors M36, M37, M38, M39, ispowered through a transistor M35 and drives a PMOS transistor M42through an inverter M40, M41. So long as the voltage Vcoscill is lessthan a voltage drop VR7, the PMOS transistor M42 stays ‘on’, and thebase current is supplied through the resistor Rb1 only. On the otherhand, as the comparator 25 switches over, the PMOS transistor M42 isturned off, and the base current of the bipolar transistor Q1 is set bythe series of resistors Rb1 and Rb2.

Plotted in FIGS. 5A, 5B and 5C are some signals present in the circuitportion of FIG. 4. In particular the voltage Vcoscill and currents Ib,Ib′ are shown. FIGS. 6 and 7 show waveforms of voltage Vc, current Icand current Ib, respectively, plotted versus time, for driver circuitsof a resistive load, with and without the reducing or optimizingelectronic circuit of this invention.

These graphs bring out that the appearance of a step due to theadditional current Ib′ is effective to drastically reduce the voltagedrops across the power device 2 during the turn-off phase. It can befurther seen from the graphs that an appropriate decrease in the basecurrent shortly before the turn-off is beneficial in terms of chargestoring and voltage drop without affecting performance at turn-on.

FIGS. 8 and 9 show waveforms of voltage Vc, current Ic and current Ib,respectively, plotted versus time, during the turn-off phase. Thesegraphs bring out as a suitable reduction of the base current, close tothe turn-off phase, provides benefits in terms of charge storage andvoltage drop while maintaining the circuit performances during theturn-on phase.

From the foregoing it will be appreciated that, although specificembodiments of the invention have been described herein for purposes ofillustration, various modifications may be made without deviating fromthe spirit and scope of the invention. Accordingly, the invention is notlimited except as by the appended claims.

That which is claimed is:
 1. An electronic circuit for supplying a loadand having reduced switching losses, the electronic circuit comprising:a first current generator producing a first drive current; astructurally independent second current generator producing a seconddrive current; a switching power device for connecting to the load andhaving at least one control terminal; a first switch between saidstructurally independent second current generator and said at least onecontrol terminal of said switching power device; and a control circuitcontrolling said first switch during turn-on and off phases of saidswitching power device.
 2. An electronic circuit according to claim 1wherein said switching power device comprises: a power transistor havingfirst and second conduction terminals, the second conduction terminalconnected to a voltage reference; and a plurality of transistorsconnected between the first conduction terminal and a power supply. 3.An electronic circuit according to claim 2 wherein said plurality oftransistors comprises first and second bipolar transistors connected ina Darlington configuration; and wherein said first bipolar transistorhas a base terminal being driven by an adjustable current signal.
 4. Anelectronic circuit according to claim 3 further comprising: an inputterminal receiving a voltage enabling signal; a plurality of resistorsconnected in series between said input terminal and said base terminal;and a second switch connected to selectively short-circuit at least oneof said resistors according to switching needs.
 5. An electronic circuitaccording to claim 3 further comprising: a frequency control block; andan over current block controlled by said frequency control block andhaving an output connected to said base terminal.
 6. An electroniccircuit according to claim 4 further comprising a frequency controlblock driving said second switch and the control terminal of said powertransistor thus setting a conduction duration thereof.
 7. An electroniccircuit according to claim 2 wherein said power transistor comprises aMOS transistor.
 8. An electronic circuit according to claim 2 furthercomprising a gate driver block having an output connected to the controlterminal of said power transistor and an input receiving a voltageenabling signal.
 9. An electronic circuit according to claim 8 furthercomprising a frequency control block having an output connected to thecontrol terminal of said power transistor and an input receiving thevoltage enabling signal.
 10. An electronic circuit for supplying a loadand having reduced switching losses, the electronic circuit comprising:a first current generator producing a first drive current; a secondcurrent generator producing a second drive current; a switching powerdevice for connecting to the load and having at least one controlterminal; and a controller for selectively connecting said secondcurrent generator to the at least one control terminal of said switchingpower device during at least one of a turn-on and off phase of saidswitching power device.
 11. An electronic circuit according to claim 10wherein said controller comprises: a first switch between said secondcurrent generator and said at least one control terminal of saidswitching power device; and a control circuit controlling said firstswitch.
 12. An electronic circuit according to claim 10 wherein saidswitching power device comprises: a power transistor having first andsecond conduction terminals, the second conduction terminal connected toa voltage reference; and a plurality of transistors connected betweenthe first conduction terminal and a power supply.
 13. An electroniccircuit according to claim 12 wherein said plurality of transistorscomprises first and second bipolar transistors connected in a Darlingtonconfiguration; and wherein said first bipolar transistor has a baseterminal being driven by an adjustable current signal.
 14. An electroniccircuit according to claim 13 further comprising: an input terminalreceiving a voltage enabling signal; a plurality of resistors connectedin series between said input terminal and said base terminal; and asecond switch connected to selectively shortcircuit at least one of saidresistors according to switching needs.
 15. An electronic circuitaccording to claim 13 further comprising: a frequency control block; andan over current block controlled by said frequency control block andhaving an output connected to said base terminal.
 16. An electroniccircuit according to claim 14 further comprising a frequency controlblock driving said second switch and the control terminal of said powertransistor thus setting a conduction duration thereof.
 17. An electroniccircuit according to claim 12 wherein said power transistor comprises aMOS transistor.
 18. An electronic circuit according to claim 12 furthercomprising a gate driver block having an output connected to the controlterminal of said power transistor and an input receiving a voltageenabling signal.
 19. An electronic circuit according to claim 18 furthercomprising a frequency control block having an output connected to thecontrol terminal of said power transistor and an input receiving thevoltage enabling signal.
 20. An electronic circuit for supplying a loadand having reduced switching losses and comprising: a first currentgenerator producing a first drive current; a second current generatorproducing a second drive current; a switching power device forconnecting to the load and having at least one control terminal; and acontroller for selectively connecting said first and second currentgenerators to the at least one control terminal of said switching powerdevice during turn-on and off phases of said switching power device. 21.An electronic circuit according to claim 20 wherein said controllercomprises: a first switch between said first current generator and saidat least one control terminal of said switching power device; a secondswitch between said second current generator and said at least onecontrol terminal of said switching power device; and a control circuitcontrolling said first and second switches.
 22. An electronic circuitaccording to claim 21 wherein said switching power device comprises: apower transistor having first and second conduction terminals, thesecond conduction terminal connected to a voltage reference; and aplurality of transistors connected between the first conduction terminaland a power supply.
 23. An electronic circuit according to claim 22wherein said plurality of transistors comprises first and second bipolartransistors connected in a Darlington configuration; and wherein saidfirst bipolar transistor has a base terminal being driven by anadjustable current signal.
 24. An electronic circuit according to claim23 further comprising: an input terminal receiving a voltage enablingsignal; a plurality of resistors connected in series between said inputterminal and said base terminal; and a third switch connected toselectively short-circuit at least one of said resistors according toswitching needs.
 25. An electronic circuit according to claim 23 furthercomprising: a frequency control block; and an over current blockcontrolled by said frequency control block and having an outputconnected to said base terminal.
 26. An electronic circuit according toclaim 24 further comprising a frequency control block driving said thirdswitch and the control terminal of said power transistor thus setting aconduction duration thereof.
 27. An electronic circuit according toclaim 22 wherein said power transistor comprises a MOS transistor. 28.An electronic circuit according to claim 22 further comprising a gatedriver block having an output connected to the control terminal of saidpower transistor and an input receiving a voltage enabling signal. 29.An electronic circuit according to claim 28 further comprising afrequency control block having an output connected to the controlterminal of said power transistor and an input receiving the voltageenabling signal.
 30. A method for supplying a load using an electroniccircuit having reduced switching losses, the electronic circuitcomprising a switching power device having at least one controlterminal, the method comprising: producing a first drive current;producing a second drive current; and selectively supplying the firstand second drive currents to the at least one control terminal of theswitching power device during at least one of a turn-on and off phase ofthe switching power device.
 31. A method according to claim 30 whereinthe switching power device comprises a power transistor having first andsecond conduction terminals, the second conduction terminal connected toa voltage reference; and first and second bipolar transistors connectedin a Darlington configuration between the first conduction terminal anda power supply; and further comprising: driving a base terminal of thefirst bipolar transistor by an adjustable current signal.
 32. A methodaccording to claim 31 wherein the power transistor comprises a MOStransistor.